Small World-based Query Mechanism

Zhiqiang Liu1, Lifang Wang2, Zhike Zhang2, Aihua Zhang1, Zejun Jiang2 1. College of software and microelectronics, Northwestern Polytechnical University, Xian China, 710072 2. School of Computer Science and Technology, Northwestern Polytechnical University, Xian China, 710072

Email: zqliu@nwpu.edu.cn, wanglf@nwpu.edu.cn, zhangzhike@mail.nwpu.edu.cn, aihua.cherry@hotmail.com, claud@nwpu.edu.cn

Abstract—the majority of routing protocols for wireless sensor networks attempt to obtain optimal or shortest paths that lead to target resources. It is energy inefficient to search and establish those paths. CZQueen, a Small World-based resource query mechanism, is presented to reduce energy overhead of resource queries in large-scale and location-free sensor networks. Based on the Small World Model, CZQueen uses contacts as shortcuts to reduce the average path length of networks. The mechanism introduces Tight-Zone to cut down the number of shortcuts, and gives relative positions of vicinage nodes to achieve a sense of direction without location information in queries. We execute a set of simulation experiments used to evaluate performance of CZQueen. Analytical performance evaluation shows that CZQueen achieves high performance in workload balance with the same algorithmic complexity as CAPTURE. The simulation results indicate that CZQueen has lower energy overhead compared to ZRP and CAPTURE, which is robust and scalable as well.

Keywords- query mechanism; Small World; Contact; sensor networks; overhead

I. INTRODUCTION Wireless sensor networks are designed as self-configuring,

infrastructure-less, unattended, rapidly-deployable networks. It is composed of a large number of nodes with low-cost, small size, limited power and transmission range. These tiny nodes have very limited computation, storage, and communication capability. The features described above make resource query a challenging problem in wireless sensor networks.

Sensor network can be viewed as distributed sense database. One of the main functions of such networks is to resolve queries and carry out transactions and small transfers. So, resource query product a large proportion of network traffic.

This paper introduces CZQueen query mechanism, a location-free, energy efficient query mechanism based on the concept of small worlds [1] [2]. The main design goal in such target applications is to reduce communication overhead and power consumption, rather than route optimization. CZQueen is suitable for resource discovery as well as routing very small data transfers in which the cost of data transfer is much smaller than the cost of route discovery.

II. RELATED WORK Small world theory [1-2] has shown that adding a few

numbers of random links, called shortcuts, to regular graphs results in graphs with low average path length and high clustering. Many people are devoted to the applicability of the small world concept to wireless multi-hop networks, including ad-hoc and sensor networks. E.g. Gaurav Sharma investigate the use of wired link act as shortcuts to bring down the average hop count of wireless sensor network [3]. Prof. Ahmed Helmy introduce concept of contacts (based on the concept of small-world) to enhance nodes' view and aid in route and resource discovery in [4], in that architecture, nodes within a limited number of hops from each node form the vicinity of that node. Resources within the vicinity can be readily accessed with the help of a proactive scheme within the vicinity. For accessing resources beyond the vicinity, each node also maintains a few distant nodes by special method, those distant nodes called contacts. Contacts act as short cuts to transform the wireless network into a small world and hence reduce the average degrees of separation between the query source and the target. Subsequently, based on concept of contacts, Prof. Ahmed Helmy presented CARD [5-6], TRANSFER [7], and MARQ [8] protocol for large-scale ad-hoc networks.

CZQueen is on the basis of production of CARD and TRANSFER protocol, and laid a strong emphasis on improves on contact selection mechanism. In the rest of this paper, by analysis of CARD and TRANSFER protocol, CZQueen will be presented as an elaborate case study for the design, evaluation and analysis of an efficient resource discovery protocol for large-scale wireless sensor networks.

III. NETWORK MODEL AND DESIGN IDEAS

A. Network model We consider a sensor network that is present as follow: • N sensor nodes are distributed uniformly inside a

square area whose side length is Y. there isn’t hole, where can not be sensed by any sensor node, exist in the area.

• Each sensor node adopts Boolean Sensing Model, i.e. each sensor has a certain sensing range, r. all the sensors have identical circular coverage areas, and a sensor node can only sense the environment and detect events within its coverage area.

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• All the nodes remain stationary. • No sensor node is equipped with component which

can determine position, distance and direction, like GPS.

There is a kind of query in sensor networks: A query only acquires very small data (e.g., inquiring about one variable, such as temperature). One distinguishing characteristic of such queries is that the communication cost for route discovery (to resolve the query) may exceed the cost of data transfer. Thus, the idea that we reduce overall cost is to find accessible path from source to target instead of to find optimal path.

IV. CZQUEEN MECHANISM OVERVIEW

A. Definitions An overview of the CZQueen mechanism is shown in

Fig.1: Following are some terminology definitions: Neighbors (of a node): All nodes within one hop from the

node. Tight-Zone: A group of nodes that each node in the group

is neighbor of the others is a Tight-Zone, a Tight-Zone maintain a group of contacts.

Vicinity (of a Tight-Zone): All nodes within a particular number of hops (R) from the Tight-Zone. H is the radius of the vicinity.

Edge nodes (of a Tight-Zone): All nodes at a distance of H hops from the Tight-Zone.

Number of Contacts (NoC): NoC specifies the value of the maximum number of contacts to be searched for each source node.

Depth of search (D): D specifies the levels of contacts (i.e., contacts of contacts) queried by a source.

B. Establishing Tight-Zone

Tight-Zone is established as follow: 1) Each node broadcast a hello message. Every node

discovers Neighbors by received hello messages, as shown in Fig 2(a).

2) Each node broadcast its Neighbor information. Every node receives all of Neighbors’ Neighbors information, as

shown in Fig 2(b). 3) If a node isn’t a member of any Tight-Zone, and can find one or above of its Neighbors which do not belong to any Tight-Zone, it became head node of Zone randomly. 4) Head node establishes Tight-Zone, as shown in Fig 2(c), and broadcasts Tight-Zone information to its Neighbors.

C. Maintaining Vicinity information

Every Tight-Zone collects and stores information of its vicinities within R hops away. The vicinity is maintained using a proactive localized link state protocol. Tight-Zone distinguishes edge nodes from all of its vicinities. As shown in Fig 3(a), node 2 is node 1’s neighbor, and node 3 is node 2’s neighbor, and isn’t node 1’ neighbor. Therefore, node 3 locates in grey area. By this approach, Tight-Zone can learn relatively position of all its edge nodes. As shown in Fig 3(b). Each Tight-Zone divides its edge nodes into NoC number of groups that named edge node group.

Each Tight-Zone also maintains state for (a few) nodes

that lie outside its vicinities. These nodes serve as contacts for accessing resources beyond the vicinities.

Contact Selection starts when a node S within a Tight-Zone sends a Contact Selection (CS) message through each of its edge node group. CS message include: 1. ID of S; 2. list of edge nodes of S; 3. d, number of hops from S; 4. List of middle node from S to target node.

When a node receive a CS, lets d=d+1. if 2H≤d≤Y/2, the node become Contact with probability

/ 2CdRPY

= . C≥1 is

constant. If the node did not become Contact, the node forwards CS to one of its edge nodes which locate in the opposite direction to the CS come from.

① ② ③

(a) (b) Fig. 3. Determine position of edge nodes

Fig. 2. Establishing Tight-Zone.

H

R

Fig. 1. Overview of CZQUEEN mechanism.

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D. Search Policy Search Policies of contact-based architecture include

Single-shot Policy [6] and Level-by-level Policy [7]. Single-shot Policy will be analyzed in this paper. In Single-shot Policy, the request is sent out from source node once in a single attempt. The request is forwarded directly from level-1 contacts to level-2 contacts, up to level-D contacts. As shown in Fig 4.

V. EVALUATION AND ANALYSIS OF CZQUEEN NS-2 [9] along with CZQueen and its extensions was used

to generate various scenarios of wireless sensor networks. Our simulations did not consider MAC-layer issues.

Here we present results for a topology of N nodes uniformly spread over area of Ym×Ym, node transmission range is 110m. H=3，NoC=6. Values of N and Y are listed in table I.

TABLE I. VALUES OF N AND Y

N Y(m)

200 1000

500 1400

1000 2000

2000 2800

4000 3700

First we try to understand the effect of various parameters such as number of nodes and Depth of search D.

Fig.5 illustrates the Query Success Rate for different number of nodes and D. As number of nodes increases, the Query Success Rate decreases. The Query Success Rate increases along with D increases.

Fig.6 illustrates the energy overhead for different network scale. Results in Fig.6 show that energy overhead increases when network size and D increase.

We compare the performance of CZQueen to that of

CARD in terms of average overall overhead.

Simulations were repeated several times with varying

source-target node pairs which is selected randomly in the network. The same pairs were used for those mechanisms. Fig.10. shows the average overall overhead (measured by traffic) for random queries with different network sizes. According to this figure and analysis of all above, we can draw a conclusion that CZQueen outperforms CAPTURE and ZRP in overhead and query success rate.

Fig. 7 Energy overhead for CZQueen, CAPTURE and ZRP.

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r of P

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Que

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ss R

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Fig 4 Search Policy

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VI. CONCLUSIONS In this paper we investigate the issues of contact-based

architecture, CZQueen, for resource discovery in large-scale wireless sensor networks. Contacts act as shortcuts to transform wireless sensor network into a Small-World and hence reduce the average degrees of separation between query source and target. Due to the lack of direction, CARD architecture contact selection mechanism begets a mass of backtracks overhead and curve route from source node to contact. CZQueen is on the basis of production of CARD and CAPTURE protocol, and improve on contact selection method of those protocols by make node have a sense of direction. CZQueen architecture is suitable for resource discovery as well as routing very small data transfers in which the cost of data transfer is much smaller than the cost of route discovery. As a result, compare with CAPTURE and ZRP was found to result in more query success rate and less overhead.

ACKNOWLEDGMENT This work was supported by the Shaanxi province NSF

grants 2009JQ8021 and 2009JM8017, and by aviation science foundation grant 2009ZD53044.

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